In magnetic recording/reproducing systems in which data has been recorded on a magnetic record medium in a series of discrete tracks, the problems of maintaining a magnetic transducer in the optimum transducing position over each track, i.e., tracking, during the reproduction of data has long existed. Imperfect tracking is usually a product of a combination of many factors. Some of the more noteworthy ones are physical instability, irregularities or dimensional changes in the magnetic record medium; differences between the critical tracking-determining dimensions of the machine used to make a recording and those of the machine used to reproduce it; dimensional changes or irregularities in the recording/reproducing machine; and alterations in the track configuration. Imperfect or mistracking often leads to non-repeatability of a recorded track and commonly results in the quality of the reproduced signal being degraded severely. The problem is particularly compounded when previously recorded signals must be reproduced track after track where changes in track configuration may occur.
Failure to follow or repeat track-by-track exactly a recorded track frequently occurs in helical scan video tape record and/or reproduce machines where a video signal is recorded on magnetic tape in a series of discrete parallel tracks diagonally across the tape by one or more heads. It has often been true that as a reproduce head in a helical machine scans the recorded tracks, the head will deviate significantly from the center line of each track, seriously mistracking, and, thereby, reproducing a degraded form of the recorded signal.
Various systems have been proposed in the prior art to position a magnetic head optimally with respect to a track along a record medium. In U.S. Pat. No. 3,838,453, a system is disclosed to place a scanning magnetic head over the center line of each track of recorded data. Tracking reference signals located at the beginning of each track are detected at the beginning of each scan of a data track. A servo system responsive to the sensed tracking reference signals operates to compensate for an off-track condition by controlling a capstan drive motor to adjust the magnetic tape position relative to the scanning magnetic head. While this sytem may correctly locate the scanning head relative to the track at the beginning of the track scan, if the track is not perfectly straight or does not follow a predictable path, the scanning head will deviate from the optimum transducing position over the track as it is scanned. Consequently, such systems are not suitable for use in applications, such as in helical scan machines, where tracking corrections must be made during the entire head scan of a track in order to insure that the optimum transducing position is maintained throughout the scan.
Other systems relying upon alteration of the medium transport to control the relative transducer-to-medium position are described in U.S. Pat. Nos. 3,663,763 and 3,748,408. In some of these medium transport control positioning techniques, control track information separately recorded from the data is reproduced to obtain control signals for adjusting the tension of the record medium to maintain proper tracking by the transducer (U.S. Pat. No. 3,748,408). In others of these medium transport control position techniques, data reproduced from the record medium by a transducer whose tracking is to be controlled is monitored to provide a control signal for adjusting the transport of the record medium to maintain proper tracking by the transducer (U.S. Pat. No. 3,663,763). Altering the speed of transport of the record medium has the undesirable tendency of altering the time base of data reproduced from tracks recorded in the direction of the transport of the record medium. Furthermore, techniques which rely upon the control of the transport of the record medium to maintain proper tracking by the transducer are not suitable for precise control of the transducer position relative to paths along the record medium, particularly, where large displacements (0.05 cm) of transducer/record medium position may be required at high rates (200 deflection cycles per second) to maintain proper tracking by the transducer.
Various other systems have been proposed in the prior art to position a magnetic head optimally with respect to a track along a record medium. In U.S. Pat. No. 3,246,307, a reproduce head is positioned over a track prior to reproducing recorded data. This is accomplished by a head composed of two separate magnetic elements. The head is moved until equal signals are reproduced by each element of the reproduce head. At this time, the head is properly positioned and the head positioner relinquishes control to allow normal reproduction of the recorded signal. In U.S. Pat. No. 3,292,168, two sensor heads are located on either side of the reproduce head adjacent to a data track. In a similar fashion as the system described in the U.S. Pat. No. 3,246,307, the reproduce head is positioned prior to normal reproduction of the recorded signal by the use of a positioning servo. The positioning servo is stopped when zero error difference is represented by the sensor head signals. Systems using control tracks and reproduce head vibration have also been employed for tracking purposes. An example of such systems is described in U.S. Pat. No. 3,526,726. However, none of the systems of that kind have provided during reproduction of recorded data continuous error-free head positioning along the entire length of a data track from head position information derived solely from the recorded data. Nor are such systems particularly suited for reproducing a continuous signal from a series of discrete tracks where the tracks are scanned by a transducer rotating at a substantial speed relative to the record medium.
A system for vibrating a magnetic head about its tracking path as it scans a recorded track along a magnetic disc is described in U.S. Pat. No. 3,126,535. As described therein, a fixed frequency oscillator is coupled to provide oscillatory motion to the head. This causes an amplitude modulation of the reproduced data, which is detected and utilized to position the head over the center of a track. That system includes two discrete correction channels, one for each possible direction of head position error and includes means to disable the head positioning mechanism when track center is located. Therefore, this system, too, is primarily concerned with initially locating a transducer with respect to the center line of a data track rather than continuously maintaining optimum transducing position of the transducer during the entire scan of a track. Such systems are suited for use in data record and/or reproduce systems having long term track configuration stability or where the track configuration stability requirements are not critical.
None of these tracking systems is well suited for continuously maintaining a data transducer in optimum transducing relationship with respect to a moving record medium as the transducer scans it at a high speed, such as in a helical scan video tape recorder where the magnetic record/reproduce head or heads are mounted on a rotating assembly. Furthermore, such prior art tracking systems are particularly unsuited for television recording purposes inasmuch as slightest tracking errors cause objectionable effects in the displayed television signal. In other uses, less than correct tracking may provide suitable accuracy for recovering non-visual data. However, complete recovery of data-type information is important and to such extent the present invention is useful in non-visual data recovery systems.